1. Field of the Invention
The present invention relates generally to the preparation of structural ceramic materials, and more particularly to the preparation of ceramic-ceramic prepregs for use in the manufacture of ceramic-ceramic composite articles.
Structural ceramic materials are utilized in a variety of high temperature applications which may also require high strength. For example, structural ceramics are utilized as high temperature insulating materials in the construction of rocket motors and as fan blades, combustion chambers, and after burners in the construction of jet engines. They may also find use as structural panels and members in advanced aerospace vehicles.
Heretofore, structural ceramics have usually been produced by one of two processing techniques. In a first technique, referred to as hot press sintering, ceramic powders, such as alumina, silicon carbide, boron nitride, and the like, are subjected to very high pressures and temperatures, typically in the range from about 1500.degree. C. to 2100.degree. C. While the ceramics produced by such hot press processes are generally satisfactory, the high temperature required will often degrade any reinforcement materials present. For that reason, hot press ceramics are not normally reinforced and therefore possess limited fracture toughness and strength.
Reinforced ceramic composites have often been prepared by chemical vapor deposition (CVD) of a ceramic precursor material onto a fiber reinforcement substrate. The precursor material is converted into corresponding ceramic by conventional techniques, and the process has the advantage that the reinforcement fibers are not exposed to excessively high temperatures. The process, however, is limited in that CVD often results in a non-uniform penetration of the reinforcement substrate, leading to non-uniform properties in the resulting composite product.
A relatively new technique for producing ceramic-ceramic composites involves the use of a liquid ceramic precursor material. A ceramic or other reinforcement substrate is first impregnated with a low viscosity liquid precursor, and the precursor is then converted to the corresponding ceramic matrix, typically by high temperature conversion or pyrolysis. The temperatures and pressures involved, however, are substantially lower than those required for the hot press sintering and the likelihood of degradation of the substrate is substantially reduced.
The use of low viscosity liquid ceramic precursor materials in the preparation of ceramic-ceramic composite articles has generally been limited to "wet lay-up" techniques where the reinforcement substrate is impregnated with the low viscosity ceramic precursor material and the impregnated substrate is substantially immediately cured to set the precursor prior to shaping of the substrate. Shaping of the substrate and conversion of the precursor to the corresponding ceramic material is then performed with minimum delay in order to produce the final composite article.
Although generally workable, the use of such wet lay-up techniques for producing ceramic-ceramic composites suffers from several limitations. First, reinforcement substrate impregnated with low viscosity ceramic precursor materials are very difficult to handle and shape. Such difficulties have limited the geometries which can be formed by these techniques. Second, it has been generally difficult to control the penetration of relatively low viscosity precursor materials into reinforcement substrates, frequently leading to non-uniform and/or incomplete penetration. Such non-uniform penetration can result in a product having non-uniform properties and in some cases reduce the strength of the final product. This is a particular problem with composites having relatively high fiber volumes, where the ability to maintain uniform fiber volume is particularly limited. Finally, wet lay-up technology frequently leaves voids in the composite matrix which have a deleterious effect on final product strength.
For these reasons, it would be desirable to provide methods for controlling the amount and uniformity of a ceramic precursor material which is introduced to each ply of a reinforcement substrate to maintain a constant substrate to matrix ratio. It would be further desirable to provide such methods where highly uniform penetration of a reinforcement substrate with a liquid precursor material can be achieved, even where the final product will have a relatively high fiber volume. The impregnated substrates will provide a material system which can be used for forming ceramic-ceramic substrates where the manipulation and shaping of the composite article is facilitated.
2. Description of the Background Art
U.S. Pat. No. 4,837,230, the disclosure of which is incorporated herein by reference, describes the fabrication of ceramic-ceramic composite materials using wet lay-up techniques as described above. Copending application Ser. No. 07/166,958, the disclosure of which is incorporated herein by reference, describes the production of aluminum oxide ceramic composites using similar wet lay-up techniques.